Fan or blower powered exhaust or ventilating systems are often used in industrial plants to remove and sometimes process contaminated gases produced by the machine or process being ventilated. Exhausted gases, sometimes after being processed, may be passed to the atmosphere or, after appropriate filtering and the like, recycled for further use in connection with the machine or process as make-up air. A wide variety of such machines or processes undergo a repetitive operational cycle and emit air or other gases at variable rates at different parts of the cycle. These gases may be generated by the machine or process itself or may constitute ventilating or flushing gases introduced to the machine or process. By way of example, casting machines intermittently feed molten metal into molds and require a high volume of ventilating air during the pour. Injection molding machines often use air-assisted molding which generates a high volume of exhaust air for a brief portion of the operational cycle of the machine.
Typical prior art systems have used exhaust blowers sized to exhaust the maximum volume of gases produced by the machine or process at any time during its cycle and operating on a continuous or semi-continuous basis. This requires relatively expensive and energy inefficient exhaust systems and typically requires larger make-up air units than might otherwise be required. For example, a machine might require continuous exhausting at X cubic feet per minute. Periodically, however, the exhaust volume must be increased to a much higher rate of Y cubic feet per minute, for a short period of time. Previous gas exhaust systems would be sized to continuously handle Y feet per minute. The need obviously exists for an improved system capable of meeting the exhaust needs of the machine without continuously operating at the highest exhaust rate required by the machine or process.